refactor: adopt markdown-style checkbox buffer format (#20)

* refactor(config): change default category from Inbox to Todo

* refactor(views): adopt markdown checkbox line format

Problem: task lines used an opaque /ID/  [N] prefix format that was
hard to read and inconsistent between category and priority views.
Header lines had no visual marker distinguishing them from tasks.

Solution: render headers as '## Cat', task lines as
'/ID/- [x|!| ] description'. State encoding: [x]=done, [!]=urgent,
[ ]=pending. Both views use the same construction.

* refactor(diff): parse and reconcile markdown checkbox format

Problem: parse_buffer matched the old '  text' indent pattern and
detected headers via '^%S'. Priority was read from a '[N] ' prefix.
apply() never reconciled status changes written into the buffer.

Solution: match '- [.] text' for tasks and '^## ' for headers.
Extract state char to derive priority (! -> 1) and status (x -> done).
apply() now reconciles status from the buffer, setting/clearing 'end'
timestamps — enabling the oil-style edit-checkbox-then-:w workflow.

* refactor(buffer): update syntax, extmarks, and render for checkbox format

Problem: syntax patterns matched the old indent/[N] format; right_align
virtual text produced a broken layout in narrow windows; the done
strikethrough skipped past the '  ' indent leaving '- [x] ' unstyled;
render() added undo history entries so 'u' could undo a re-render.

Solution: update taskHeader/taskLine patterns for '## '/'- [.]'; rename
taskPriority -> taskCheckbox matching '[!]'; switch virt_text_pos to
'eol'; drop the +2 col_start offset so strikethrough covers '- [x] ';
guard nvim_buf_set_lines with undolevels=-1 so renders are not undoable.
Also fix open_line to insert '- [ ] ' and position cursor at col 6.

* refactor(init): replace multi-level priority with binary toggle

Problem: <C-a>/<C-x> overrode Vim's native number increment and the
visual g<C-a>/g<C-x> variants added complexity for marginal value.
toggle_complete() left the cursor on the wrong line after re-render.

Solution: remove change_priority/change_priority_visual; add
toggle_priority() (0<->1) mapped to '!', with cursor-follow after
render matching the pattern already used in priority toggle. Add
cursor-follow to toggle_complete() for the same reason. Update plugin
plugs (priority-up/down -> priority) and add 'due'/'undo' to the
:Pending completion list. Update help text accordingly.

* feat(buffer): reflect current view in buffer name

Problem: no way to tell at a glance which view (category vs priority)
is active — the buffer was always named 'pending://'.

Solution: update the buffer name to 'pending://category' or
'pending://priority' on every render, so the view is visible in
the statusline/tabline without any extra UI.

lua/pending/views.lua

@@ -125,7 +125,7 @@ function M.category_view(tasks)
       table.insert(lines, '')
       table.insert(meta, { type = 'blank' })
     end
-    table.insert(lines, cat)
+    table.insert(lines, '## ' .. cat)
     table.insert(meta, { type = 'header', category = cat })
 
     local all = {}
@@ -138,9 +138,8 @@ function M.category_view(tasks)
 
     for _, task in ipairs(all) do
       local prefix = '/' .. task.id .. '/'
-      local indent = '  '
-      local prio = task.priority > 0 and ('[' .. task.priority .. '] ') or ''
-      local line = prefix .. indent .. prio .. task.description
+      local state = task.status == 'done' and 'x' or (task.priority > 0 and '!' or ' ')
+      local line = prefix .. '- [' .. state .. '] ' .. task.description
       table.insert(lines, line)
       table.insert(meta, {
         type = 'task',
@@ -189,9 +188,8 @@ function M.priority_view(tasks)
 
   for _, task in ipairs(all) do
     local prefix = '/' .. task.id .. '/'
-    local indent = '  '
-    local prio = task.priority == 1 and '! ' or ''
-    local line = prefix .. indent .. prio .. task.description
+    local state = task.status == 'done' and 'x' or (task.priority > 0 and '!' or ' ')
+    local line = prefix .. '- [' .. state .. '] ' .. task.description
     table.insert(lines, line)
     table.insert(meta, {
       type = 'task',